1. Field of the Invention
This invention relates to a power amplifier system for a radio transmitter, and more specifically to a linear power amplifier system suitable for use in a mobile communication terminal. In particular, the present invention is concerned with a burst controller and a burst control method for the power amplifier system.
2. Description of the Related Art
QAM (Quadrature Amplitude Modulated) signals are now used for services such as mobile telephone systems. Elements which are important for the transmission of information by means of these QAM signals include not only their frequency but also their amplitude. Linear amplification is therefore essential upon power amplification of QAM signals.
For linear amplification of power, a linear amplifier such as a class-A amplifier can be used by way of example. It is however generally difficult to obtain good power efficiency by using a linear amplifier. Application of such a linear amplifier to a mobile radio receiver/transmitter which is primarily required to be driven by batteries is therefore disadvantageous.
Accordingly, a proposal has already been made on a circuit which makes use of a non-linear amplifier capable of providing a good power efficiency and, as an entire amplifier system, can realize linear amplification.
Such a conventional circuit will hereinafter be described. The power amplifier system includes a saturated power amplifier which is a non-linear amplifier. This amplifier has a control terminal to which a control signal can be applied. The amplifier changes its output power in accordance with the voltage of the control signal. Since this amplifier is a saturated amplifier of the type that a non-linear relationship exists RF inputted signal to the amplifier and a RF signal to be outputted from the amplifier, an envelope feedback control is applied to the amplifier so that envelope signals of both a RF signal inputted into the amplifier and a RF signal to be outputted from the amplifier can be taken out. The envelope signal of the RF signal to be outputted can be linearly amplified relative to the envelope signal of the inputted RF signal.
Incidentally, the TDMA (Time Division Multiple Access) system is used for the digital mobile telephone system presently employed in North America. A radio transmitter is therefore required to transmit signals at a predetermined interval. Accordingly, ON-OFF control, namely, burst control is needed for the power amplifier system. Upon performing burst control, it is necessary to allow the power level to rise and fall as smoothly as possible. High frequency components are contained in abrupt rise and fall waveforms, so that the transmission spectrum expands at the moment of each rise or fall. This causes interference with the adjacent channels.